Tube assembly for installation into a duct

ABSTRACT

A tube assembly ( 1 ) comprises a dummy tube ( 2 ) formed from extruded medium density polyethylene and five primary tubes ( 3 ), also formed from extruded medium density polyethylene, and placed around the dummy tube ( 2 ). The primary tubes ( 3 ) are also provided with a co-extruded lining (not shown) on the internal surface thereof to minimise static attraction and friction between the tubes ( 3 ) and optical fibre units (not shown) subsequently installed into the tubes ( 3 ). The tubes ( 2, 3 ) and a rip cord ( 4 ), for subsequently removing the covering layers of the assembly to gain access to the tubes ( 2, 3 ), are surrounded by an outer sheath ( 5 ) formed from extruded medium density polyethylene. The outer sheath ( 5 ) is provided with a non-metallic water barrier ( 6 ), and is surrounded by a lubricant polymer layer ( 7 ) containing a polyethylene based lubricant compound, the active constituent of the lubricant being an organic silicone slip agent.

The present invention relates to tube assemblies for installation into aduct, and relates particularly, but not exclusively to tube assembliesfor carrying optical fibre cables for installation into undergroundducts.

Many communications network operators have installed ducts of relativelysmall diameter (generally known to persons skilled in the art as“sub-ducts”) directly into the ground or into larger, main ducts. Thesesub-ducts are usually made of high density polyethylene and typicallyare of size 50/40 mm, 40/33 mm, 32/28 mm and 25/20 mm (i.e. outsidediameter/inside diameter). Also, many older ducts are of size 50/40 mmand 40/33 mm.

It has been desirable to sub divide these ducts by installing smallertubes, either as a bundle of tubes encapsulated in a sheath or asseveral individual tubes. The disadvantage with the first of thesemethods is that encapsulating the tubes as a bundle produces an itemwhich is relatively stiff. This problem is made worse by the fact thatit is desirable for the encapsulating sheath to be manufactured from amaterial with a relatively low coefficient of friction. It is well knownto persons skilled in the art that such low friction materials aremanufactured primarily from high density polyethylene, and it is a goodgeneral rule that the higher the density the lower the friction. It isalso a good general rule that the higher the density the stiffer thematerial. The problem of encapsulating the tubes producing a stifferproduct is therefore aggravated by the additional stiffness resultingfrom the high density, low coefficient of friction jacket. As a result,such assemblies will not blow very far, particularly in tortuous routes.The need to divide sub ducts exists primarily in metropolitan areas, soroutes involve many road crossings and therefore many bends.

An attempt to overcome this problem has been made by blowing individualtubes. The tubes themselves are much more flexible that the assembly.However, this process suffers from the drawback that it is relativelycomplicated. It is necessary to have multiple drums on site holding theindividual tubes. The installation of tubes by blowing is in factachieved by a combination of pushing and blowing. The pushing device isusually a caterpillar device which exerts a degree of pressure on thetubes being pushed. The individual tubes are not protected by a sheathand are relatvely small and fragile. It is therefore necessary tointernally pressurise the individual tubes so that they do not collapseunder the pressure of the caterpillar pushing device. Despite this, thetubes are still damaged on occasion during the installation process,making subsequent placement of fibre optic cable into the individualtubes unreliable.

Preferred embodiments of the present invention seek to overcome theabove disadvantages of the prior art.

According to the present invention, there is provided a tube assemblyfor installation into a duct, the assembly comprising:

at least one hollow flexible tube;

a first layer enclosing the or each said flexible tube and having aflexural modulus of less than 400 Megapascals (Mpa); and

a lubricating layer surrounding said fast layer;

characterised in that at least one said hollow flexible tube is adaptedto have at least one respective flexible signal transmitting memberinstalled therein subsequently to installation of the assembly into aduct.

By providing a first layer having the flexibility set out above and alubricating layer surrounding the first layer, this provides theadvantage of simultaneously avoiding the problems encountered in blowingstiffer/lower friction (such as high density polyethylene) and moreflexible/higher friction (such as medium density polyethylene) materialsaround curves, while having a coefficient of friction comparable withmaterial such as high density polyethylene. This solution also avoidsthe need to internally pressurise the tubes and provides a protectivesheath against the compressive forces of the caterpillar pushing device.In particular, low friction materials such as high density polyethylene,while encountering less frictional resistance against the wall of thesub-duct during blowing, are stiffer than higher friction materials, asa result of which difficulty is encountered in blowing tubes of thematerial around curves. Higher friction materials are more flexible andtherefore encounter less difficulty in passing around curves, butencounter more frictional resistance to blowing.

The assembly of the present invention can be blown surprisingly far andrapidly in comparison with the arrangements of the prior art.Furthermore, there is a strong prejudice in the relevant art againstattempting to blow several tubes simultaneously, blowing of each tubeseparately being considered the only method possible. The presentinvention therefore has the advantage that installation rates aresignificantly greater than in prior art arrangements.

The first layer may have a flexural modulus of less than 350 Megapascals(Mpa).

At least one said hollow tube may be formed from polyethylene.

In a preferred embodiment, said first layer is formed from polyethylene.

The first layer may be substantially circular in external cross-section.

The first layer may be substantially polygonal in externalcross-section.

The first layer may have a thickness of between 0.5 mm and 3 mm.

At least one said hollow tube may be adapted to receive at least oneoptical fibre.

The lubricating layer may include an organic silicone material.

The organic silicone material may be a slip agent.

The lubricating layer may include an antistatic material.

The antistatic material may be an amine based material.

The assembly is preferably free of ductile materials.

By providing an assembly free of ductile materials, this provides theadvantage of avoiding the use of materials which will plastically deformduring installation or when wrapped around a storage drum, and whichwould thus cause a kink in the assembly which would obstructinstallation.

A preferred embodiment of the above invention will now be described, byway of example only and not in any limitative sense, with reference tothe accompanying drawing in which:

FIG. 1 is a schematic cross-sectional illustration of an optical fibrecable embodying the invention.

Referring to FIG. 1, a tube assembly 1 (such as high densitypolyethylene) comprises a dummy tube 2 of 7 mm external diameter formedfrom extruded medium density polyethylene and five primary tubes 3 of 10mm external diameter and 8 mm internal diameter, also formed fromextruded medium density polyethylene, and placed around the dummy tube2. The medium density polyethylene used to form the dummy tube 2 andprimary tubes 3 is typically in the density range 0.927 to 0.94 g percm³, and the primary tubes 3 are also provided with a co-extruded lining(not shown) on the internal surface thereof to minimise staticattraction and friction between the tubes 3 and optical fibre units (notshown) subsequently installed into the tubes 3. The tubes 2, 3 and a ripcord 4, for subsequently removing the covering layers of the assembly togain access to the tubes 2, 3, are surrounded by an outer sheath 5formed from extruded medium density polyethylene density 0.939 g percm³, for example available from BP Chemicals Limited, whose registeredaddress is Britannic House, 1 Finsbury Circus. London 7BA, UnitedKingdom, as grade number V20D760S. The outer sheath 5, which has athickness of between 0.5 mm and 3 mm is provided with a non-metallicwater barrier 6, and is surrounded by a lubricant polymer layer 7containing a polyethylene based lubricant compound, the activeconstituent of the lubricant being an organic silicone slip agent.

The cable assembly 1 of FIG. 1 has unexpectedly been found to beinstallable over significantly greater distance by blowing than theseparate tubes of the prior art.

EXAMPLE

A cable assembly differing from that of FIG. 1 only in that the waterbarrier 6 has been removed was blown into a 50/40.8 mm ribbed duct bymeans of a compressed air blowing machine of a type which will befamiliar to persons skilled in the art. The results of the blowingoperation were as follows Time (minutes) Hydraulic Pressure Air Pressure0.0 45 Bar   6 Bar 2.0 50 Bar   7 Bar 4.0 50 Bar 7.5 Bar 6.0 60 Bar   8Bar 8.0 70 Bar 8.5 Bar 10.0 60 Bar 8.5 Bar 10.56 Cable out

It was found that the cable travelled a total distance of 940 metres in10 minutes 56 seconds, representing an average installation speed of 85metres per minute.

COMPARATIVE EXAMPLE

A similar blowing test was carried out on an assembly which differedfrom the assembly 1 of FIG. 1 in that the lubricant layer 7 was absent,an aluminium foil was present as the water barrier 6, and the mediumdensity polyethylene outer sheath 5 was applied more loosely than in thecase of assembly 1 of FIG. 1. Such a product is known for the purpose ofbeing pulled into duct, but is not suitable for being blown into a ductby means of compressed air.

The results of the tests were as follows Distance (m) Speed HydraulicPressure Air Pressure 226 40 m per min 100 Bar 8 Bar 354 55 m per min100 Bar 8 Bar 510 40 m per min 100 Bar 8 Bar 622 35 m per min 110 Bar 8Bar 716 35 m per min 110 Bar 8 Bar

The cable stopped after a distance of 760 m as a result of theinstallation apparatus slipping on the outer sheath, and then stoppedagain at 870 m.

It can therefore be seen that the present invention can be installedwith significantly higher blowing speeds and over significantly greaterdistances than in the case of the prior art.

It will be appreciated by persons skilled in the art that the aboveinvention has been described by way of example only and not in anylimitative sense, and that various alterations and modifications arepossible without departure from the scope of the invention as defined bythe appended claims.

1-14. (cancelled)
 15. A tube assembly for installation into a duct, theassembly comprising: at least one hollow flexible tube; a first layerenclosing the or each said flexible tube and having a flexural modulusof less than 400 Megapascals (Mpa); and a lubricating layer surroundingsaid first layer; characterised in that at least one said hollowflexible tube is adapted to have at least one respective flexible signaltransmitting member installed therein subsequently to installation ofthe assembly into a duct.
 16. An assembly according to claim 15, whereinsaid first layer has a flexural modulus of less than 350 Megapascals(Mpa).
 17. An assembly according to claim 15, wherein at least one saidhollow tube is formed from polyethylene.
 18. An assembly according toclaim 15, wherein said first layer is formed from polyethylene.
 19. Anassembly according to claim 15, wherein the first layer is substantiallycircular in external cross-section.
 20. An assembly according to claim15, wherein the first layer is substantially polygonal in externalcross-section.
 21. An assembly according to claim 15, wherein the firstlayer has a thickness of between 0.5 mm and 3 mm.
 22. An assemblyaccording to claim 15, wherein at least one said hollow tube is adaptedto receive at least one optical fibre.
 23. An assembly according toclaim 15, wherein the lubricating layer includes an organic siliconematerial.
 24. An assembly according to claim 23, wherein the organicsilicone material is a slip agent.
 25. An assembly according to claim15, wherein the lubricating layer includes an antistatic material. 26.An assembly according to claim 25, wherein the antistatic material is anamine based material.
 27. An assembly according to claim 15, wherein theassembly is free of ductile materials.